1. Field of the Invention
This invention resides in the field of propulsion systems with variable thrust, and in particular to nozzles that utilize pintles to vary the nozzle throat area.
2. Description of the Prior Art
Mechanisms for thrust variation have long been used to provide rocket motors with a relatively high thrust at the boost stage and a low thrust during the sustain phase. Early efforts to achieve this type of boost-sustain transition focused on the configuration of the solid propellant grain to provide a high burning surface area in the early stages of burning while the shape of the propellant grain by itself caused a reduction in the burning surface area as burning progressed. The variability produced by such a design was limited however and not controllable during flight, and the shape of the propellant grain was often optimal for only one of the phases, usually the boost phase. Subsequent efforts focused on the throat of the motor nozzle by introducing various for changing the effective throat area. A decrease in the effective throat area raises the pressure upstream of the throat and thereby increases the thrust. Elevated pressure also increases the burn rate of the propellant, adding further to the thrust. This ability to vary the effective throat area allowed the use of variable thrust to be extended to include steering and orientation adjustments during flight in addition to the boost-sustain transition.
Numerous systems have been designed to vary the throat area by means that are independent of the burning stage of the propellant and controllable from outside the nozzle. These systems are useful for both boost-sustain transitions and for steering and orientation, and can be used in both single-thruster and multiple-thruster motors. A mechanism for throat area variation in a rocket nozzle that has proved to be particularly successful is the use of a pintle that extends into the nozzle and is movable along the nozzle axis. The pintle is either a tapered or flared body that partially obstructs the throat, forcing the combustion gas to flow in the annular space between the pintle and the throat wall. Because of the pintle profile, movement of the pintle by only a small distance causes a significant change in the cross section area of the annular space and hence the effective throat area of the nozzle and therefore the thrust. When multiple nozzles with independently movable pintles are present, a controller can coordinate the pintle positions to produce different thrust levels among the nozzles to achieve thrust differentials for purposes of steering, attitude control and directional effects in general. Movement of the pintle is achieved by actuators that employ any of a variety of mechanisms, including hydraulic drives and gear drives.
One of the challenges that are faced in the design of a pintle-actuated thrust chamber is the need to control the wear on and damage to the pintle actuator that are caused by the exposure of the actuator to the harsh conditions of the thrust chamber, notably the high temperatures and pressures in the chamber. The wear that these conditions produce on the actuator components causes the actuator to deteriorate and thereby limit the duty cycle of the thruster. The highest temperature occurs in the vicinity of the throat, and heat is readily transmitted from the throat along the pintle body to the actuator. Another challenge is the need for a dynamic seal between the pintle and the thrust chamber that will both retain the pressure and accommodate the movement of the pintle. In designs where the pintle actuator is incorporated in the thrust chamber, the dynamic seal resides in the thrust chamber as well, with little or no insulation protecting the seal from the high heat at the throat. In these designs as well, the chamber must be large enough to enclose the actuator and therefore consumes valuable space within the propulsion system. This has been partially remedied by placing the actuator external to the thrust chamber, with the seal mounted in the thrust chamber wall. Even in the wall, however, the seal is still exposed to the high temperature of the thrust chamber and the resulting deterioration limits the useful life span of the seal.